DataGridTextColumn.MaxLength?

Question

How do I set the MaxLength property of the DataGridTextColumn?

Answer 1

If you have a shared style among all columns, and you would like to add an additional style to one or more of those, you could use the Style.BasedOn Property:

 <DataGridTextColumn Binding="{Binding SomeProperty, UpdateSourceTrigger=PropertyChanged}" ElementStyle="{StaticResource CellErrorStyle}">
     <DataGridTextColumn.EditingElementStyle>
           <Style TargetType="TextBox" BasedOn="{StaticResource OriginalStyleKey}">
                <Setter Property="MaxLength" Value="5" />                            
           </Style>
     </DataGridTextColumn.EditingElementStyle>
 </DataGridTextColumn>

Answer 2

<tk:DataGridTextColumn Binding="{Binding Text}">
    <tk:DataGridTextColumn.EditingElementStyle>
        <Style TargetType="TextBox">
            <Setter Property="MaxLength" Value="16"/>
        </Style>
    </tk:DataGridTextColumn.EditingElementStyle>
</tk:DataGridTextColumn>

You could also set it using the following behavior so you don't have to use a style and setters each time:

Public Class TextBoxBehavior
    Private Shared Types As Type() = New Type() {GetType(AutoCompleteBox), GetType(ComboBox), GetType(DataGridTextColumn)}

    Public Shared Function GetMaxLength(ByVal element As DependencyObject) As Integer
        Return element.GetValue(MaxLengthProperty)
    End Function

    Public Shared Sub SetMaxLength(ByVal element As DependencyObject, ByVal value As Integer)
        element.SetValue(MaxLengthProperty, value)
    End Sub

    Private Shared Sub ValidateElement(ByVal element As DependencyObject)
        If element Is Nothing Then Throw New ArgumentNullException("element")
        If Not Types.Contains(element.GetType) Then Throw New NotSupportedException("The TextBoxBehavior is not supported for the given element")
    End Sub

    Public Shared ReadOnly MaxLengthProperty As DependencyProperty = 
     DependencyProperty.RegisterAttached("MaxLength", 
                        GetType(Integer), GetType(TextBoxBehavior), 
                        New FrameworkPropertyMetadata(Integer.MaxValue, AddressOf TextBox_MaxLengthChanged))

    Private Shared Sub TextBox_MaxLengthChanged(ByVal sender As Object, ByVal e As DependencyPropertyChangedEventArgs)
        If sender Is Nothing Then Exit Sub

        Dim value = DirectCast(e.NewValue, Integer)

        If TypeOf sender Is AutoCompleteBox Then
            Dim acb = DirectCast(sender, AutoCompleteBox)

            If acb.IsLoaded Then
                Dim tb = DirectCast(acb.Template.FindName("Text", acb), TextBox)
                tb.MaxLength = value
            Else
                acb.AddHandler(AutoCompleteBox.LoadedEvent, New RoutedEventHandler(AddressOf Element_Loaded))
            End If
        ElseIf TypeOf sender Is ComboBox Then
            Dim cb = DirectCast(sender, ComboBox)
            If cb.IsLoaded Then
                Dim tb = DirectCast(cb.Template.FindName("PART_EditableTextBox", cb), TextBox)
                tb.MaxLength = value
            Else
                cb.AddHandler(ComboBox.LoadedEvent, New RoutedEventHandler(AddressOf Element_Loaded))
            End If
        ElseIf TypeOf sender Is DataGridTextColumn Then
            Dim dgtc = DirectCast(sender, DataGridTextColumn)

            Dim setter = GetIsMaxLengthSet(dgtc.EditingElementStyle)
            If setter Is Nothing Then
                Dim style = New Style(GetType(TextBox), dgtc.EditingElementStyle)
                style.Setters.Add(New Setter(TextBox.MaxLengthProperty, value))
                dgtc.EditingElementStyle = style
                style.Seal()
            Else
                setter.Value = value
            End If
        End If
    End Sub

    Private Shared Function GetIsMaxLengthSet(ByVal style As Style) As Setter
        If style Is Nothing Then Return Nothing
        Dim setter = style.Setters.LastOrDefault(Function(s) TypeOf s Is Setter AndAlso DirectCast(s, Setter).Property Is TextBox.MaxLengthProperty)
        If setter IsNot Nothing Then Return setter Else Return GetIsMaxLengthSet(style.BasedOn)
    End Function

    Private Shared Sub Element_Loaded(ByVal sender As Object, ByVal e As RoutedEventArgs)
        Dim ml = GetMaxLength(sender)
        TextBox_MaxLengthChanged(sender, New DependencyPropertyChangedEventArgs(TextBox.MaxLengthProperty, -1, ml))
        sender.RemoveHandler(FrameworkElement.LoadedEvent, New RoutedEventHandler(AddressOf Element_Loaded))
    End Sub
End Class

Usage:

<ComboBox xmlns:loc="MyNamesapace" loc:TextBoxBehavior.MaxLength="50" />